This application claims the priority benefit of Japanese application serial no. 2001-269010 filed on Sep. 5, 2001.
1. Field of Invention
The present invention relates to a method for producing nitrogen by using a pressure swing adsorption (PSA) method with the air as a raw material, wherein a carbon molecular sieve (CMS) is particularly used as an adsorbent in order to produce high purity nitrogen gas. More particularly, the present invention relates to a method and an apparatus for producing nitrogen with a PSA method that uses a CMS adsorbent having suitable adsorption properties as evaluated with an index, which is for evaluating the gas separating ability of the adsorbent required for improving the performance of nitrogen production with PSA.
2. Description of Related Art
In recent years, regarding the methods for producing nitrogen, a method that uses an adsorbent preferentially adsorbing oxygen to produce high purity nitrogen gas from a gas mixture of oxygen and nitrogen, such as the air, with a PSA method has been widely adopted. The carbon molecular sieve (CMS), as its name indicates, is an activated carbon that has the function of molecular sieving, and features with a smaller mean pore size and a sharper pore size distribution as compared with general activated carbon. Since the pore size of CMS is very close to the molecular size of the adsorbate, the adsorption rates are lower for certain adsorbent/adsorbate combinations. For example, in the case that a nitrogen-rich gas is obtained from a gas mixture of oxygen and nitrogen like the air, the CMS preferably has an adsorption rate for nitrogen much lower than that for oxygen because of the difference between the molecular sizes of oxygen and nitrogen.
A PSA method comprises the following steps. In an adsorption step wherein a raw gas is compressed under a proper pressure and then introduced into an adsorbing column formed with an adsorbent filling layer therein, the easy to adsorb component is adsorbed preferentialy and the difficult to adsorb component is collected as a product. When the adsorbent is saturated with the easy to adsorb component after the adsorption step is performed for a certain period, the raw gas supplied into the adsorbing column formed with the adsorbent layer therein in the adsorption step is switched off. The adsorbing column is opened to the atmosphere to lower the pressure in the column to the atmospheric pressure, so as to desorb the easy to adsorb component from the adsorbent for the regeneration of the adsorbent. By applying the above operation from a single column to a plurality of columns with the adsorption step and the regeneration step being repeated periodically, the product is obtained continuously.
Similarly, a pressurization step, an adsorption step, a depressurization step and a regeneration step are performed sequentially in an apparatus that uses CMS as an adsorbent and utilizes the phenomenon that an adsorption rate for nitrogen is much smaller than that for oxygen, i.e., utilizes the difference between the adsorption rates of the two, to produce nitrogen from an O2/N2 mixture gas such as the air. Generally, the process includes the exchange of gases between two adsorbing columns during the switch from the adsorption step to the depressurization step, and during the switch from the regeneration step to the pressurization step. Such an operation is called a xe2x80x9cpressure equalizing operation between two columnsxe2x80x9d.
To improve the performance of a nitrogen PSA apparatus, many techniques are disclosed focusing on both the adsorbent and the PSA process itself. The correlation between the adsorbent and the nitrogen PSA apparatus is disclosed in Japanese Patent Publication No. Sho 54-17595, wherein the nitrogen PSA operation uses a coke molecular sieve obtained by introducing a hydrocarbon and decomposing it with pyrolysis to release carbon, and depositing the released carbon in the pores of coke. The nitrogen PSA operation features that a raw gas is introduced with a flow-through rate of 0.01xcx9c0.04 Nm3 per second with 1 m3 of the coke molecular sieve.
Moreover, Japanese Patent Application Laid Open No. Sho 59-45914 relates to a method for fabricating a CMS used in a nitrogen PSA apparatus. The document discloses that an oxygen adsorption amount higher than 5 mL/g at equilibrium and a selectivity higher than 20xcx9c23 are preferable as being the necessary performances the adsorbent should have for effectively separating oxygen and nitrogen in the PSA process. The above values of selectivity are obtained by measuring the time needed for the sample to adsorb nitrogen to the same amount as the oxygen amount being adsorbed under 1 atm and 25xc2x0 C. for 5 seconds, and then taking the ratio of the two adsorption times, wherein the sample is kept in vacuum before adsorption. However, nothing is mentioned about the ranges of the adsorption rates for oxygen and nitrogen with the produced CMS in this application.
On the other hand, Japanese Patent Application Laid Open No. Sho 59-182215 discloses a CMS having a mean effective pore size of about 3xcx9c20 xc3x85, a required range of oxygen diffusibility and the selectivity ratio for oxygen adsorption and nitrogen adsorption. However, the range of the mean effective pore size is quite broad and the pore size suitable for separating oxygen and nitrogen is not clear in this application. Moreover, the validity of the oxygen diffusibility disclosed in the application cannot be judged because the results of using the nitrogen PSA apparatus are not described.
Japanese Patent Application Laid Open No. Hei 3-232515 describes the use of a CMS that has adsorption amounts of 20xcx9c27 mg/g and 1xcx9c6 mg/g for oxygen and nitrogen, respectively, under an adsorption pressure of 245 kPa (gauge pressure) after 1 minute, and has an adsorption amount of 22xcx9c34 mg/g for the two at equilibrium. The application also disclosed that the adsorption time in the high-pressure adsorption step is 130xcx9c300 seconds and the outflow rate (L/min) of the nitrogen product is 0.1xcx9c3.0 times the volume (L) of the adsorbing column in the PSA operation. In this application, the properties of the used adsorbent and the limitation of the range of the ratio of xe2x80x9cthe product outflow rate to the volume of the adsorbing columnxe2x80x9d, which is the characteristic value of the PSA operation, are decided according to the concentration of the oxygen gas contained in the nitrogen product.
As described in the aforementioned prior art, many techniques are disclosed about the CMS adsorbent and the PSA process relating to a nitrogen PSA apparatus. The factors suitably indicating the properties of a CMS adsorbent, including the adsorbed amounts of oxygen and nitrogen at equilibrium and the ratio thereof, the adsorption rates for oxygen and nitrogen and the ratio thereof, and the adsorption times for oxygen and nitrogen and the ratio thereof, are proposed. However, no established evaluating method and indicator have been obtained so far and some problems have consequently occurred. Moreover, some market requirements, such as further miniaturization (compactification) of the nitrogen PSA apparatus and high purity ( greater than 99.99%) of the product, are not sufficiently satisfied in practice.
In view of the aforementioned issues, this invention provides a new indicator capable of indicating the performance of a CMS. This invention also provides a PSA method that greatly improves the performance of the conventional nitrogen-producing apparatuses within a wide purity range of the nitrogen product from 99% to 99.999% by using a CMS adsorbent having a suitable performance index.
To solve the aforementioned issues, this invention provides a method for producing nitrogen by using a pressure swing adsorption (PSA) method with the air as a raw material, wherein the adsorbent used is a carbon molecular sieve (CMS) for selectively adsorbing oxygen. The time xe2x80x9cTOxe2x80x9d needed for the CMS to adsorb an oxygen amount of 50% of the saturated oxygen adsorption amount starting from oxygen supply is 5xcx9c10 seconds. The time xe2x80x9cTNxe2x80x9d needed for the CMS to adsorb a nitrogen amount of 50% of the saturated nitrogen adsorption amount starting from nitrogen supply is larger than TO by more than 41 times. The air is made to contact with the CMS and a pressure swing adsorption (PSA) process is performed to produce nitrogen. The method is operated in a state such that the production rate of nitrogen with one ton of adsorbent is higher than 100 Nm3/h(our), 150 Nm3/h or 250 Nm3/h as the oxygen concentration in the nitrogen product is 100 ppm, 1000 ppm or 10000 ppm, wherein the oxygen concentration serves as a purity indicator of the nitrogen product. Meanwhile, the method is operated in a state such that the processing rate of the raw air with one ton of adsorbent is lower than 500 Nm3/h, 570 Nm3/h or 690 Nm3/h as the oxygen concentration in the nitrogen gas product is 100 ppm, 1000 ppm or 10000 ppm, wherein the oxygen concentration serves as a purity indicator of the nitrogen product.
The apparatus for producing nitrogen of this invention comprises an air compressor, a dryer, at least one adsorbing column and a product tank. The air compressor is used to compress the air and the dryer to remove water from the compressed air. The adsorbing column is filled with an adsorbent capable of selectively adsorbing oxygen, while the compressed air is introduced into the adsorbing column for selective adsorption of oxygen after water is removed. The product tank is used to temporarily store the nitrogen product conducted out of the adsorbing column after oxygen is removed in the adsorbing column. In the adsorbing column, an adsorption step and a depressurization regeneration step are alternatively and periodically switched to implement a pressure swing adsorption (PSA) process. The adsorption step comprises compressively introducing the raw air into the adsorbing column, and the depressurization regeneration step comprises releasing the pressurized gas in the adsorbing column after the adsorption step. The adsorbent is a carbon molecular sieve (CMS) capable of selectively adsorbing oxygen, which adsorbs an oxygen amount of 50% of the saturated oxygen adsorption amount with a period xe2x80x9cTOxe2x80x9d of 5xcx9c10 seconds starting from oxygen supply, and adsorbs a nitrogen amount of 50% of the saturated nitrogen adsorption amount with a period xe2x80x9cTNxe2x80x9d larger than TO by more than 41 times starting from nitrogen supply.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.